The present invention relates generally to satellite stationkeeping systems and methods, and more particularly, to such systems and methods that use an electric propulsion system to simultaneously correct orbital inclination and eccentricity growth and use a chemical propulsion system to correct orbital semimajor axis growth.
It is generally well known in the art that various forces act on synchronous satellites to move the satellite out of stationary orbit. These forces are due to several sources including the gravitational effects of the sun and moon, the nonspherical shape of the Earth and solar radiation pressure. To counter these forces, synchronous satellites are equipped with propulsion systems that are fired at intervals in order to maintain station at a desired longitude. This requires control of the orbital inclination, eccentricity and semimajor axis of the satellite. Inclination is a measure of the north-south position of the satellite relative to the Earth's equator. Eccentricity is the measure of the noncircularity of the satellite orbit. That is, the measure of the variation of the distance the satellite is from the earth as the earth and satellite rotate. Finally, the semimajor axis is a measure of the average position of the satellite relative to the center of the earth.
Current three-axis stabilized satellites use separate sets of thrusters to control north-south and east-west motions. The north thrusters produce the required north-south change in satellite velocity, or delta V, to control orbit inclination. The east thrusters and west thrusters produce the required combined east-west delta V to control orbit semimajor axis and eccentricity. For each of these three maneuvers, thrusters are fired in pairs to cancel torques since the thrust directions do not pass through the satellite center of mass. Furthermore, there are three separate maneuvers performed at different times as required by the physics of the various forces. The frequency of these maneuvers are typically every 14 days for both the north-south maneuver and the pair of east-west maneuvers (east and west firings occur approximately 1/2 orbit apart or about 12 hours) when using 5 pound thrusters with liquid propulsion.
U.S. Pat. No. 5,443,231 entitled "Method And Apparatus For A Satellite Station Keeping" discloses the nominal use of four electric propulsion thrusters to simultaneously correct for orbital inclination, eccentricity, and semimajor axis growth of a satellite orbiting in a synchronous orbit about a central body such as the earth. This is illustrated in FIG. 1 of the cited patent. This requires the thrusters to be canted in the east-west direction by an angle .alpha. of at least 10 degrees. This is illustrated in FIG. 2 of the cited patent. It has been determined that the use of method and apparatus disclosed in U.S. Pat. No. 5,443,231 could lead to a significant mass and power penalty in case of a thruster failure (as high as 52% for a satellite mass of 3000 kilograms, electric propulsion Isp of 1620 seconds, and solar array area of 150 square meters). This failure requires extra maneuvers as illustrated in FIG. 3 of the cited patent.
Accordingly, it would be an advantage to have satellite stationkeeping systems and methods that improves upon the teachings of U.S. Pat. No. 5,443,231 and maximizes the payload mass and mission life of satellites in which they are employed.